Automatic failover for domain controller

Solution 1:

Active Directory uses DNS SRV records to locate domain controllers. Unless you've taken the time to set up a different AD Site where that other DC lives, chances are your clients are already using that other domain controller. When one DC is down, there will be some timeouts as clients attempt to talk to the dead DC but will automatically connect to any other DC in the domain.

Active Directory is a multi-master database. Unlike Windows NT, the concept of Primary and Backup DCs is no longer valid. All domain controllers can serve all authentication requests at all times. DNS is used to list the valid DCs, and clients pick the DC to talk to from that list.

Solution 2:

What you really have is 2 Domain Controllers installed - the concept of Primary and Backup Domain Controllers died long ago. They are both acting as Domain Controllers without any intervention from you.

In short, you've already done what you intended by running dcpromo on the subsequent Domain Controller.

This article describes how it works in Windows XP - I don't think the process has changed that much with later Operating Systems.